A silicon carbide semiconductor (SiC) has a large band gap as compared to a silicon semiconductor (Si) and therefore, has a high critical dielectric field intensity. ON-resistance is resistance in a conductive state and is inversely proportional to the cube of the critical dielectric field intensity. Therefore, for example, widely used silicon carbide semiconductors called a 4H type can suppress the ON-resistance to a few hundredths as compared to silicon semiconductors. The silicon carbide semiconductors also have large thermal conductivity characteristics facilitating heat dissipation. As described above, SiC is expected as a next-generation, low-loss power semiconductor element and silicon carbide semiconductor elements having various structures are developed as Schottky barrier diodes, MOSFETs, PN diodes, IGBTs, GTOs, etc.
Among these elements, Schottky barrier diodes are unipolar devices and therefore, have very small reverse recovery current at the time of turn-off and is expected to replace Si-pin diodes.
Fabrication of a Schottky barrier diode will hereinafter roughly be described. First, an n-type epitaxial layer having a film thickness of 10 μm and a donor concentration of 1×1016 cm−3 is grown and formed on a low-resistance n-type 4H-SiC substrate. A p-type well region having a concentration on the level of 1017 cm−3 is formed into a ring shape on the n-type epitaxial layer. On the outside of this p-type well region, a p-type well region having a concentration lower than the p-type well region may be disposed. On the outside of the low-concentration p-type well region, multiple low-concentration p-type well regions may further be disposed. The p-type well region is formed by Al ion implantation and annealing at a high temperature greater than or equal to 1600 degrees C. The p-type well region is referred to as an element termination structure and has a function of alleviating an electric field at the termination end of the element to prevent deterioration in breakdown voltage.
In some elements, multiple p-type well regions having an accepter concentration of 1×1018 cm−3 or higher may partially be formed at predetermined intervals in an internal region surrounded by the p-type well region on the n-type epitaxial layer. This structure is referred to as a junction barrier Schottky (JBS) diode. Since an n-type region disposed between p-type well regions can be pinched off at the time of reverse bias, this structure can advantageously reduce a leak current in the reverse direction. A distance between the adjacent p-type well regions is set to a dimension of several μm so that the n-region disposed between the p-type wells can be pinched off.
Subsequently, an oxide film patterned to exclude a portion of the p-type well region and the inside region thereof is formed, and Schottky metal is formed to overhang the oxide film. Although an ohmic electrode may be formed on a surface of the p-type well region of JBS, this increases the number of steps leading to an increase in manufacturing costs and, therefore, the surface of the p-type well region of JBS is generally coated with the Schottky metal to achieve Schottky contact. Subsequently, Al metal, polyimide, and a back surface metal (back surface electrode) are sequentially formed to complete the Schottky diode.
When the positive bias that is applied to an anode electrode of the Al metal of the Schottky diode fabricated as described above is increased, a forward current accordingly increases and a reaction (silicidation or carbonization of Ti) eventually occurs in the interface between the Schottky metal and SiC due to heat generation thereby, which reduces a Schottky barrier height. This allows more current to flow and self-heat-generation applies positive feedback facilitating the interface reaction, eventually leading to thermal destruction. The forward current in this case is referred to as a surge current. A wide band gap semiconductor is disclosed that has improved resistance to such a surge current (see. e.g., Patent Document 1).
Japanese Laid-Open Patent Publication No. 2011-151208